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On 3.2 subprocess.Popen.communicate with universal_newlines=True accepts bytes and doesn't accept strings. 
$ ./python -c "import subprocess; subprocess.Popen(['cat'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, universal_newlines=True).communicate('qwerty')"
Traceback (most recent call last):
 File "<string>", line 1, in <module>
 File "/home/serhiy/py/cpython3.2/Lib/subprocess.py", line 833, in communicate
 return self._communicate(input)
 File "/home/serhiy/py/cpython3.2/Lib/subprocess.py", line 1470, in _communicate
 stdout, stderr = self._communicate_with_poll(input)
 File "/home/serhiy/py/cpython3.2/Lib/subprocess.py", line 1537, in _communicate_with_poll
 input_offset += os.write(fd, chunk)
TypeError: 'str' does not support the buffer interface
On 3.3+ it accepts strings and doesn't accept bytes.